Synthetic amorphous silica powder and method for producing same

ABSTRACT

The synthetic amorphous silica powder of the present invention is characterized in that it comprises a synthetic amorphous silica powder obtained by applying a spheroidizing treatment to a silica powder, and by subsequently cleaning and drying it so that the synthetic amorphous silica powder has an average particle diameter D 50  of 10 to 2,000 μm; wherein the synthetic amorphous silica powder has: a quotient of 1.00 to 1.35 obtained by dividing a BET specific surface area of the powder by a theoretical specific surface area calculated from the average particle diameter D 50 ; a real density of 2.10 to 2.20 g/cm 3 ; an intra-particulate porosity of 0 to 0.05; a circularity of 0.75 to 1.00; and an unmolten ratio of 0.00 to 0.25. This synthetic amorphous silica powder is less in amount of gas components adsorbed to surfaces of particles of the powder and in amount of gas components within the particles, so that a synthetic silica glass product manufactured by using the powder is remarkably decreased in amount of generation or degree of expansion of gas bubbles even upon usage of the product in a high temperature and reduced pressure environment.

TECHNICAL FIELD

The present invention relates to a synthetic amorphous silica powderwith high purity and a method for producing the same, which silicapowder is suitable as a raw material for manufacturing a syntheticsilica glass product such as a piping, crucible, or the like to be usedin a high temperature and reduced pressure environment in asemiconductor industry and the like.

BACKGROUND ART

Conventionally, crucibles, jigs, and the like to be used for singlecrystal production in semiconductor application have been manufacturedfrom a quartz powder as a raw material obtained by pulverizing andpurifying a natural quartz, quartz sand, or the like. However, thenatural quartz, quartz sand, or the like contains various metalimpurities which are not completely removed therefrom even by thepurification treatment, so that the thus obtained raw material has notbeen fully satisfied in purity. In turn, progression of high integrationof semiconductors has led to a more enhanced quality demand for singlecrystals as materials for the semiconductors, so that crucibles, jigs,and the like to be used for producing the single crystals are alsodemanded to have high purities, respectively. As such, attention hasbeen directed to a synthetic silica glass product obtained by adopting,as a raw material, a synthetic amorphous silica powder with high purityinstead of a natural quartz, quartz sand, or the like.

As a method for producing such a synthetic amorphous silica powder withhigh purity, there has been disclosed a method where silicontetrachloride with high purity is hydrolyzed with water, and thegenerated silica gel is dried, sized, and fired, to obtain a syntheticamorphous silica powder (see Patent Document 1, for example). Further,methods have been disclosed each configured to hydrolyze an alkoxysilanesuch as silicate in the presence of acid and alkali to thereby gelatethe alkoxysilane, and to dry and pulverize the obtained gel, followed byfiring to thereby obtain a synthetic amorphous silica powder (see PatentDocuments 2 and 3, for example). The synthetic amorphous silica powdersproduced by the methods described in the Patent Documents 1 to 3,respectively, are highly pure as compared to a natural quartz, quartzsand, and the like, thereby exemplarily enabling to: decrease entrainedimpurity contaminations in synthetic silica glass products such ascrucibles, jigs, and the like manufactured from such synthetic amorphoussilica powders as raw materials, respectively; and enhance performancesof the products, respectively.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent Publication No. 4-75848 (Claim 1)

Patent Document 2: Japanese Patent Application Laid-Open Publication No.62-176929 (Claim 1)

Patent Document 3: Japanese Patent Application Laid-Open Publication No.3-275527 (page 2, lower left column, line 7 to page 3, upper leftcolumn, line 6)

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, those synthetic silica glass products manufactured from thesynthetic amorphous silica powders as raw materials produced by themethods described in the Patent Documents 1 to 3, respectively, eachhave a drawback that the applicable synthetic silica glass productgenerates gas bubbles or the gas bubbles are expanded when the syntheticsilica glass product is used in a high temperature and reduced pressureenvironment, in a manner to considerably deteriorate the performance ofthe synthetic silica glass product.

For example, crucibles for silicon single crystal pulling are each usedin an environment at a high temperature of about 1,500° C. and at areduced pressure of about 7,000 Pa, such that the considerabledeterioration of a performance of the crucible due to the aforementionedgeneration or expansion of gas bubbles has been a problem affecting aquality of pulled a single crystal.

Against the above problem to be brought about upon usage in a hightemperature and reduced pressure environment, it is conceivable toconduct such a countermeasure to lower a concentration of thoseimpurities in a synthetic amorphous silica powder which possibly act asgas components: by applying a heat treatment to a synthetic amorphoussilica powder obtained by hydrolysis of silicon tetrachloride, tothereby decrease respective concentrations of hydroxyl group andchlorine in the synthetic amorphous silica powder; or, by applying aheat treatment to a synthetic amorphous silica powder obtained fromalkoxysilane by a sol-gel method, to thereby decrease respectiveconcentrations of hydroxyl group and carbon in the synthetic amorphoussilica powder.

However, even by the above countermeasure, it is impossible tosufficiently restrict generation or expansion of gas bubbles in asynthetic silica glass product to be used in a high temperature andreduced pressure environment.

It is therefore an object of the present invention to provide asynthetic amorphous silica powder and a method for producing the same,which silica powder overcomes the conventional problem and is suitableas a raw material for manufacturing a synthetic silica glass productwhich is less in amount of generation or degree of expansion of gasbubbles even upon usage of the product in a high temperature and reducedpressure environment.

Means for Solving Problem

The first aspect of the present invention resides in a syntheticamorphous silica powder obtained by applying a spheroidizing treatmentto a granulated silica powder, and by subsequently cleaning and dryingit so that the synthetic amorphous silica powder has an average particlediameter D₅₀ of 10 to 2,000 μm; characterized in that the syntheticamorphous silica powder has:

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25.

The second aspect of the present invention resides in an invention basedon the first aspect, characterized that the synthetic amorphous silicapowder is obtained by applying the spheroidizing treatment to thegranulated silica powder after firing it; and

that the synthetic amorphous silica powder satisfies one or both of theconditions that it has a carbon concentration less than 2 ppm and thatit has a chlorine concentration less than 2 ppm.

The third aspect of the present invention resides in an invention basedon the second aspect, characterized in that the granulated silica powderis a silica powder obtained by: hydrolyzing silicon tetrachloride toproduce a siliceous gel; drying the siliceous gel to turn it into a drypowder; pulverizing particles of the dry powder; and then classifyingthe pulverizedly obtained particles; and

that the synthetic amorphous silica powder has a carbon concentrationless than 2 ppm.

The fourth aspect of the present invention resides in an invention basedon the second aspect, characterized in that the granulated silica powderis a silica powder obtained by: hydrolyzing an organic silicon compoundto produce a siliceous gel; drying the siliceous gel to turn it into adry powder; pulverizing particles of the dry powder; and thenclassifying the pulverizedly obtained particles; and

that the synthetic amorphous silica powder has a chlorine concentrationless than 2 ppm.

The fifth aspect of the present invention resides in an invention basedon the second aspect, characterized in that the granulated silica powderis a silica powder obtained by: using a fumed silica to produce asiliceous gel; drying the siliceous gel to turn it into a dry powder;pulverizing particles of the dry powder; and then classifying thepulverizedly obtained particles; and

that the synthetic amorphous silica powder has a carbon concentrationless than 2 ppm and a chlorine concentration less than 2 ppm.

The sixth aspect of the present invention resides in a method forproducing a synthetic amorphous silica powder, comprising, in therecited order:

a granulating step for producing a siliceous gel, drying the siliceousgel to turn it into a dry powder, pulverizing particles of the drypowder, and then classifying the pulverizedly obtained particles tothereby obtain a silica powder;

a spheroidizing step based on a thermal plasma for delivering, at apredetermined supplying rate, particles of the silica powder obtained inthe granulating step into a plasma torch in which a plasma is generatedby a predetermined high-frequency power, in a manner to heat theparticles at a temperature from 2,000° C. to a boiling point of silicondioxide, thereby melting the particles;

a cleaning step for removing fine particles attached to surfaces of thespheroidized silica powder particles after the spheroidizing step; and

a drying step for drying the silica powder particles after the cleaningstep;

wherein the spheroidizing step is conducted by adjusting a value of A/B(W·hr/kg) to 1.0×10⁴ or more, where A is the high-frequency power (W),and B is the supplying rate (kg/hr) of the silica powder, under acondition that the high-frequency power A is 90 kW or higher, therebyobtaining a synthetic amorphous silica powder having:

an average particle diameter D₅₀ of 10 to 2,000 μm;

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25.

The seventh aspect of the present invention resides in an inventionbased on the sixth aspect, characterized in that the granulating step isa step for: hydrolyzing silicon tetrachloride to produce a siliceousgel; drying the siliceous gel to turn it into a dry powder; pulverizingparticles of the dry powder; and then classifying the pulverizedlyobtained particles, to obtain a silica powder having an average particlediameter D₅₀ of 10 to 3,000 μm.

The eighth aspect of the present invention resides in an invention basedon the sixth aspect, characterized in that the granulating step is astep for: hydrolyzing an organic silicon compound to produce a siliceousgel; drying the siliceous gel to turn it into a dry powder; pulverizingparticles of the dry powder; and then classifying the pulverizedlyobtained particles, to obtain a silica powder having an average particlediameter D₅₀ of 10 to 3,000 μm.

The ninth aspect of the present invention resides in an invention basedon the sixth aspect, characterized in that wherein the granulating stepis a step for: using a fumed silica to produce a siliceous gel; dryingthe siliceous gel to turn it into a dry powder; pulverizing particles ofthe dry powder; and then classifying the pulverizedly obtainedparticles, to obtain a silica powder having an average particle diameterD₅₀ of 10 to 3,000 μm.

The tenth aspect of the present invention resides in a method forproducing a synthetic amorphous silica powder, comprising, in therecited order:

a granulating step for producing a siliceous gel, drying the siliceousgel to turn it into a dry powder, pulverizing particles of the drypowder, and then classifying the pulverizedly obtained particles tothereby obtain a silica powder;

a firing step for firing particles of the silica powder obtained in thegranulating step, at a temperature of 800 to 1,450° C.;

a spheroidizing step based on a thermal plasma for delivering, at apredetermined supplying rate, particles of the silica powder obtained inthe firing step into a plasma torch in which a plasma is generated by apredetermined high-frequency power, in a manner to heat the particles ata temperature from 2,000° C. to a boiling point of silicon dioxide,thereby melting the particles;

a cleaning step for removing fine particles attached to surfaces of thesilica powder particles after the spheroidizing step; and

a drying step for drying the silica powder particles after the cleaningstep;

wherein the spheroidizing step is conducted by adjusting a value of A/B(W·hr/kg) to 1.0×10⁹ or more, where A is the high-frequency power (W),and B is the supplying rate (kg/hr) of the silica powder, under acondition that the high-frequency power A is 90 kW or higher, therebyobtaining a synthetic amorphous silica powder having:

an average particle diameter D₅₀ of 10 to 2,000 μm;

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25; and

wherein the synthetic amorphous silica powder satisfies one or both ofthe conditions that it has a carbon concentration less than 2 ppm andthat it has a chlorine concentration less than 2 ppm.

The eleventh aspect of the present invention resides in an inventionbased on the tenth aspect, characterized in that, when the granulatingstep is a step for: hydrolyzing silicon tetrachloride to produce asiliceous gel; drying the siliceous gel to turn it into a dry powder;pulverizing particles of the dry powder; and then classifying thepulverizedly obtained particles, to obtain a silica powder having anaverage particle diameter D₅₀ of 10 to 3,000 μm,

the obtained synthetic amorphous silica powder has a carbonconcentration less than 2 ppm.

The twelfth aspect of the present invention resides in an inventionbased on the tenth aspect, characterized in that, when the granulatingstep is a step for: hydrolyzing an organic silicon compound to produce asiliceous gel; drying the siliceous gel to turn it into a dry powder;pulverizing particles of the dry powder; and then classifying thepulverizedly obtained particles, to obtain a silica powder having anaverage particle diameter D₅₀ of 10 to 3,000 μm,

the obtained synthetic amorphous silica powder has a chlorineconcentration less than 2 ppm.

The thirteenth aspect of the present invention resides in an inventionbased on the tenth aspect, characterized in that, when the granulatingstep is a step for: using a fumed silica to produce a siliceous gel;drying the siliceous gel to turn it into a dry powder; pulverizingparticles of the dry powder; and then classifying the pulverizedlyobtained particles, to obtain a silica powder having an average particlediameter D₅₀ of 10 to 3,000 μm,

the obtained synthetic amorphous silica powder has a carbonconcentration less than 2 ppm and a chlorine concentration less than 2ppm.

Effect of the Invention

The synthetic amorphous silica powder according to the first aspect ofthe present invention is a synthetic amorphous silica powder obtained byapplying a spheroidizing treatment to a granulated silica powder, and bysubsequently cleaning and drying it so that the synthetic amorphoussilica powder has an average particle diameter D₅₀ of 10 to 2,000 μm;

wherein the synthetic amorphous silica powder has:

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25.

Thus, by using this synthetic amorphous silica powder to manufacture asynthetic silica glass product, gas components adsorbed to surfaces ofparticles of a raw powder are less in amount and gas components insidethe powder particles are also less in amount, thereby enabling to reducean amount of generation or degree of expansion of gas bubbles.

The synthetic amorphous silica powder according to the second aspect ofthe present invention is obtained by applying the spheroidizingtreatment to the granulated silica powder after firing it; wherein thesynthetic amorphous silica powder satisfies one or both of theconditions that it has a carbon concentration less than 2 ppm and thatit has a chlorine concentration less than 2 ppm. Thus, by using thissynthetic amorphous silica powder to manufacture a synthetic silicaglass product, gas components adsorbed to surfaces of particles of a rawpowder are less in amount and gas components inside the powder particlesare also less in amount, thereby enabling to reduce an amount ofgeneration or degree of expansion of gas bubbles. Particularly, in caseof this synthetic amorphous silica powder, firing is conducted beforeapplying the spheroidizing treatment to the powder, so that gascomponents to be adsorbed to surfaces of powder particles and gascomponents inside the particles are extremely decreased in amount,thereby further enhancing an effect for reducing an amount of generationor degree of expansion of gas bubbles in the above synthetic silicaglass product.

In case of the synthetic amorphous silica powder according to the fifthaspect of the present invention, the granulated silica powder is asilica powder obtained by: using a fumed silica to produce a siliceousgel; drying the siliceous gel to turn it into a dry powder; pulverizingparticles of the dry powder; and then classifying the pulverizedlyobtained particles; thereby achieving that the synthetic amorphoussilica powder has a carbon concentration less than 2 ppm and a chlorineconcentration less than 2 ppm.

In case of this synthetic amorphous silica powder, the granulated silicapowder is further restricted in carbon concentration and chlorineconcentration by using fumed silica as a silica powder as a raw powder,as compared to a granulated silica powder which uses, as a raw powder, asilica powder obtained by reacting a chlorine-based silicon compound ina liquid or a silica powder obtained from an organic silicon compoundsuch as tetramethoxysilane; thereby further enhancing an effect forreducing an amount of generation or degree of expansion of gas bubblesin the synthetic silica glass product.

In the method for producing a synthetic amorphous silica powderaccording to the sixth to ninth aspects of the present invention, thesiliceous gel is produced by hydrolyzing silicon tetrachloride, thesiliceous gel is produced by hydrolyzing an organic silicon compoundsuch as tetramethoxysilane, or the siliceous gel is produced by usingfumed silica, for example. The method is characterized in that itcomprises in the recited order:

a granulating step for drying the siliceous gel to turn it into a drypowder, pulverizing particles of the dry powder, and then classifyingthe pulverizedly obtained particles to thereby obtain a silica powderhaving a desired average particle diameter;

a spheroidizing step based on a thermal plasma for delivering, at apredetermined supplying rate, particles of the silica powder obtained inthe granulating step into a plasma torch in which a plasma is generatedby a predetermined high-frequency power, in a manner to heat theparticles at a temperature from 2,000° C. to a boiling point of silicondioxide, thereby melting the particles;

a cleaning step for removing fine particles attached to surfaces of thesilica powder particles after the spheroidizing step; and

a drying step for drying the silica powder particles after the cleaningstep;

wherein the spheroidizing step is conducted by adjusting a value of A/B(W·hr/kg) to 1.0×10⁴ or more, where A is the high-frequency power (W),and B is the supplying rate (kg/hr) of the silica powder, under acondition that the high-frequency power A is 90 kW or higher, therebyobtaining a synthetic amorphous silica powder having:

an average particle diameter D₅₀ of 10 to 2,000 μm;

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25.

The synthetic amorphous silica powder is less in inevitable gasadsorption amount and less in gas components inside the powder particlesby virtue of experience of the above steps, thereby enabling toexpediently produce a synthetic amorphous silica powder which ispreferably usable as a raw material of a synthetic silica glass product.

In the method for producing a synthetic amorphous silica powderaccording to the tenth to thirteenth aspects of the present invention,the siliceous gel is produced by hydrolyzing silicon tetrachloride, thesiliceous gel is produced by hydrolyzing an organic silicon compoundsuch as tetramethoxysilane, or the siliceous gel is produced by usingfumed silica, for example. The method is characterized in that itcomprises in the recited order:

a granulating step for drying the siliceous gel to turn it into a drypowder, pulverizing particles of the dry powder, and then classifyingthe pulverizedly obtained particles to thereby obtain a silica powder;

a firing step for firing particles of the silica powder obtained in thegranulating step, at a temperature of 800 to 1,450° C.;

a spheroidizing step based on a thermal plasma for delivering, at apredetermined supplying rate, particles of the silica powder obtained inthe firing step into a plasma torch in which a plasma is generated by apredetermined high-frequency power, in a manner to heat the particles ata temperature from 2,000° C. to a boiling point of silicon dioxide,thereby melting the particles;

a cleaning step for removing fine particles attached to surfaces of thespheroidized silica powder particles after the spheroidizing step; and

a drying step for drying the silica′ powder particles after the cleaningstep;

wherein the spheroidizing step is conducted by adjusting a value of A/B(W·hr/kg) to 1.0×10⁹ or more, where A is the high-frequency power (W),and B is the supplying rate (kg/hr) of the silica powder, under acondition that the high-frequency power A is 90 kW or higher, therebyobtaining a synthetic amorphous silica powder having:

an average particle diameter D₅₀ of 10 to 2,000 μm;

a quotient of 1.00 to 1.35 obtained by dividing a BET specific surfacearea of the powder by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀;

a real density of 2.10 to 2.20 g/cm³;

an intra-particulate porosity of 0 to 0.05;

a circularity of 0.75 to 1.00; and

an unmolten ratio of 0.00 to 0.25.

The synthetic amorphous silica powder is less in inevitable gasadsorption amount and less in gas components inside the powder particlesby virtue of experience of the above steps, thereby enabling toexpediently produce a synthetic amorphous silica powder which ispreferably usable as a raw material of a synthetic silica glass product.Particularly, this production method is configured to provide the firingstep under the predetermined condition before the spheroidizing step, ina manner to remarkably decrease amounts of gas components to beotherwise adsorbed on surfaces of powder particles and gas components tobe otherwise present within the powder particles, thereby enabling toproduce a synthetic amorphous silica powder which is higher in an effectfor reducing an amount of generation or degree of expansion of gasbubbles in the synthetic silica glass product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photographic view of representative powder particles of asynthetic amorphous silica powder according to a first embodiment of thepresent invention;

FIG. 2 is a photographic view of representative powder particles of asynthetic amorphous silica powder according to a second embodiment ofthe present invention;

FIG. 3 is a process flow diagram showing a process for producing thesynthetic amorphous silica powder according to the first embodiment ofthe present invention;

FIG. 4 is a process flow diagram showing a process for producing thesynthetic amorphous silica powder according to the second embodiment ofthe present invention;

FIG. 5 is a schematic cross-sectional view of a spheroidizing apparatusbased on thermal plasma;

FIG. 6 is a schematic view of a particle size/shape distributionmeasuring device;

FIG. 7 is a photographic view of representative silica powder particleswhich are not subjected to a spheroidizing treatment; and

FIG. 8 is a photographic view of an example of an angulate particle of asilica powder according to Comparative Evaluation 1.

MODE(S) FOR CARRYING OUT THE INVENTION

The modes for carrying out the present invention will be explainedhereinafter based on the accompanying drawings.

The synthetic amorphous silica powder according to a first embodiment ofthe present invention is obtained by applying a spheroidizing treatmentto a granulated silica powder, and by subsequently cleaning and dryingit. Further, the synthetic amorphous silica powder is characterized inthat it has: a quotient of 1.00 to 1.35 obtained by dividing a BETspecific surface area of the powder by a theoretical specific surfacearea calculated from the average particle diameter D₅₀; a real densityof 2.10 to 2.20 g/cm³; an intra-particulate porosity of 0 to 0.05; acircularity of 0.75 to 1.00; and an unmolten ratio of 0.00 to 0.25.

Further, the synthetic amorphous silica powder according to the secondembodiment of the present invention is a synthetic amorphous silicapowder, which is obtained by applying a spheroidizing treatment to agranulated silica powder after firing it. Thus, the synthetic amorphoussilica powder satisfies one or both of the conditions that it has acarbon concentration less than 2 ppm and that it has a chlorineconcentration less than 2 ppm.

Conceivable as a main source of generation or expansion of gas bubblesin a synthetic silica glass product such as a crucible for pulling up asilicon single crystal at a high temperature and a reduced pressure, isa gas adsorbed to surfaces of particles of a raw powder, which has beenused for manufacturing a product. Namely, upon manufacturing a syntheticsilica glass product, gas components adsorbed to surfaces of particlesof the raw powder are desorbed therefrom upon melting of the particlesas one step of manufacturing. Further, the thus desorbed gas componentsare left in the synthetic silica glass product, in a manner to act as asource of generation or expansion of gas bubbles.

It is typical, for a silica powder to be used as a raw material of asynthetic silica glass product, that the powder is passed through apulverizing step, so that the silica powder contains numerous particlesin indeterminate forms (pulverized powder particle forms), respectively,as shown in FIG. 7. Thus, the silica powder is considered to beincreased in specific surface area, thereby increasing an inevitable gasadsorption amount.

As such, the synthetic amorphous silica powder of the present inventionis made to have a quotient in the above range obtained by dividing a BETspecific surface area of the powder by a theoretical specific surfacearea calculated from the average particle diameter D₅₀, by applying thespheroidizing treatment to the powder. The BET specific surface area isa value measured by a BET three-point method. Further, it is possible tocalculate a theoretical specific surface area of particles from thefollowing equation (1), supposing that the particles are true spheres,respectively, and surfaces thereof are smooth. In the equation (1), Drepresents a diameter of a particle, and ρ represents a real density.

Theoretical specific surface area=6/(D×ρ)  (1)

In the present specification, the theoretical specific surface area of apowder is a value calculated from a theoretical real density, byassuming that D is an average particle diameter D₅₀ of the powder, and ρis a real density of 2.20 g/cm³ in the following equation (1). Namely,the theoretical specific surface area of a powder is calculated from thefollowing equation (2).

Theoretical specific surface area of powder=2.73/D ₅₀  (2)

Larger quotients obtained by dividing BET specific surface areas bytheoretical specific surface areas calculated from average particlediameters D₅₀, lead to larger specific surface areas, thereby increasinginevitable gas adsorption amounts. In the above, the quotient ispreferably within a range of 1.00 to 1.15. Quotients larger than 1.15lead to deteriorated effects for reducing amounts of generation ordegrees of expansion of gas bubbles.

In case that a measured value of average particle diameter D₅₀ is smallbecause a particle size distribution is skewed or a peak of particlesize distribution is present at a side where the average particlediameter D₅₀ is small, it is occasional that the quotient obtained bydividing a BET specific surface area by a theoretical specific surfacearea calculated from the average particle diameter D₅₀, becomes lessthan 1.00. In that case, the quotient obtained by dividing a BETspecific surface area by a theoretical specific surface area calculatedfrom the average particle diameter D₅₀, is determined to be 1.00.

Further, the synthetic amorphous silica powder has a circularity of 0.75or more. The circularity means that powder particles approachinglyresemble true spheres as the circularity approaches 1.00, and thecircularity is calculated from the following equation (3).

Circularity=4πS/L ²  (3)

in the equation (3), S represents an area of a particle in a projectionview, and L represents a perimeter of the particle in the projectionview. In the present specification, the circularity of the powder is anaverage value of circularities of 200 powder particles calculated fromthe equation (3). Circularities of powder less than 0.75 lead to lowereffects for reducing amounts of generation or degrees of expansion ofgas bubbles. In the above, the circularity of the powder is preferablywithin a range of 0.80 to 1.00.

Furthermore, the synthetic amorphous silica powder has an unmolten ratioof 0.25 or less. The unmolten ratio of a powder means a ratio ofangulate particles are contained in 200 particles of the powder in theprojection view thereof. Unmolten ratios greater than 0.25 lead to lowereffects for reducing amounts of generation or degrees of expansion ofgas bubbles. In the above, the unmolten ratio of the powder ispreferably within a range of 0.00 to 0.10.

Moreover, considering a single particle of the synthetic amorphoussilica powder, it is preferable that the particle is free of presence ofinterior spaces therein such as voids, closed cracks, and the like.Namely, a space(s) present inside the particle of synthetic amorphoussilica powder act(s) as a source of generation or expansion of gasbubbles in a synthetic silica glass product. As such, the real densityis to be 2.10 g/cm³ or more, and preferably 2.15 to 2.20 g/cm³. The realdensity means an average value of absolute densities obtained byconducting a real density measurement three times in conformity to JISR7212: Testing Methods for Carbon Blocks, (d) Measurement of truespecific gravity. Further, the intra-particulate porosity is to be 0.05or less, preferably 0.01 or less. The intra-particulate porosity meansan average value calculated from the following equation (4) based on 50powder particles, by measuring a cross-sectional area of each particle,and an area of a space in the particle, if present, upon observing thecross section of the particle by a SEM (scanning electron microscope):

Intra-particulate porosity=total area of spaces in particles/totalcross-sectional area of particles  (4)

In turn, the average particle diameter D₅₀ of the synthetic amorphoussilica powder is to be 10 to 2,000 μm, and preferably within a range of50 to 1,000 μm. This is because, average particle diameters smaller thanthe lower limit value lead to smaller spaces among powder particles suchthat gases present in the spaces scarcely leave therefrom and thussmaller gas bubbles are likely to be left there, while averaged particlesizes exceeding the upper limit value lead to excessively larger spacesamong powder particles such that larger gas bubbles are likely to beleft there. In the above, the average particle diameter D₅₀ isparticularly preferably within a range of 80 to 600 μm. In the presentspecification, the average particle diameter D₅₀ means an average valueof medians of particle distributions (diameter) measured three times byLaser Diffraction/Scattering Particle Size Distribution Analyzer (ModelName: HORIBA LA-950). The bulk density of the synthetic amorphous silicapowder is preferably 1.00 g/cm³ or more. This is because, bulk densitiessmaller than the lower limit value lead to excessively larger spacesamong powder particles such that larger gas bubbles are likely to beleft there, while bulk densities exceeding the upper limit value lead tosmaller spaces among powder particles such that gases present in thespaces scarcely leave therefrom and thus smaller gas bubbles are likelyto be left there. In the above, the bulk density is particularlypreferably within a range of 1.20 to 1.50 g/cm³.

To uniformalize meltabilities of powders, it is preferable that eachpowder has a broad diffraction peak and no crystalline silica powderparticles are recognized therein when the powder is measured by a powderX-ray diffractometry using a Cu—Kα line. This is because, amorphoussilica is different from crystalline silica in behavior of melting, insuch a tendency that melting of the crystalline silica is belatedlystarted; so that gas bubbles are likely to be left in a synthetic silicaglass product or the like when the synthetic silica glass product ismanufactured by a synthetic amorphous silica powder containing amorphousand crystalline silicas in a mixed manner.

So as to decrease amounts of impurities in a synthetic silica glassproduct or so as to improve a performance of the product, the syntheticamorphous silica powder is to preferably have such an impurityconcentration that concentrations are less than 1 ppm, respectively, forelements belonging to the 1A group, 2A to 8 groups, 1B to 3B groupsexcept for a hydrogen atom, for elements belonging to the 4B group and5B group except for carbon and silicon, for elements belonging to the 6Bgroup except for oxygen, and for elements belonging to the 7B groupexcept for chlorine. In the above, the impurity concentrations areparticularly preferably less than 0.05 ppm, respectively. Further, torestrict generation or expansion of gas bubbles in a synthetic silicaglass product at a high temperature and a reduced pressure, it ispreferable that a hydroxyl group, chlorine, and carbon, which possiblyact as gas components, respectively, are 60 ppm or less, 5 ppm or less,and 5 ppm or less, respectively, in concentration.

Particularly, in case of the synthetic amorphous silica powder accordingto the second embodiment of the present invention, firing is conductedbefore applying the spheroidizing treatment to the powder, so that gascomponents to be adsorbed to surfaces of powder particles and gascomponents inside the particles are extremely decreased in amount,thereby further enhancing an effect for reducing an amount of generationor degree of expansion of gas bubbles in a synthetic silica glassproduct. Namely, it is possible for the granulated silica powder tosatisfy one or both of the conditions that it has a chlorineconcentration less than 2 ppm and that it has a carbon concentrationless than 2 ppm, by firing the silica powder under a predeterminedcondition.

When the granulated silica powder, i.e., the raw powder, is a silicapowder obtained by: hydrolyzing silicon tetrachloride to produce asiliceous gel; drying the siliceous gel to turn it into a dry powder;pulverizing particles of the dry powder; and then classifying thepulverizedly obtained particles; the synthetic amorphous silica powderis made to have a carbon concentration less than 2 ppm by conductingfiring thereof under a predetermined condition before the spheroidizingtreatment. This is because, the silica powder is low in carbonconcentration as compared to a silica powder obtained by using anorganic silicon compound such as tetramethoxysilane, so that thesynthetic amorphous silica powder obtained by using the former silicapowder as a raw powder is relatively decreased in concentration ofresidual carbon.

Further, when the granulated silica powder is a silica powder obtainedby: hydrolyzing an organic silicon compound to produce a siliceous gel;drying the siliceous gel to turn it into a dry powder; pulverizingparticles of the dry powder; and then classifying the pulverizedlyobtained particles; the synthetic amorphous silica powder is made tohave a chlorine concentration less than 2 ppm by conducting firingthereof under a predetermined condition before the spheroidizingtreatment. This is because, the silica powder is low in chlorineconcentration as compared to a silica powder obtained by reacting achlorine-based silicon compound in a liquid, so that the syntheticamorphous silica powder obtained by using the former silica powder as araw powder is relatively decreased in concentration of residualchlorine.

Moreover, when the granulated silica powder is a silica powder obtainedby: using a fumed silica to produce a siliceous gel; drying thesiliceous gel to turn it into a dry powder; pulverizing particles of thedry powder; and then classifying the pulverizedly obtained particles;the synthetic amorphous silica powder is made to have a carbonconcentration less than 2 ppm and a chlorine concentration less than 2ppm by conducting firing thereof under a predetermined condition beforethe spheroidizing treatment. Namely, the synthetic amorphous silicapowder obtained by using a silica powder obtained by reacting a rawpowder with a chlorine-based silicon compound in a liquid, is likely tobecome relatively high in concentration of residual chlorine. Further,the synthetic amorphous silica powder obtained by using an organicsilicon compound as a raw powder, is likely to become relatively high inconcentration of residual carbon. In turn, the fumed silica is low inboth of chlorine concentration and carbon concentration as compared tothe above-described two types of silica powders, so that the syntheticamorphous silica powder obtained by using the fumed silica as a rawpowder is extremely decreased in both of chlorine concentration andcarbon concentration. This further enhances an effect for reducing anamount of generation or degree of expansion of gas bubbles in asynthetic silica glass product.

The synthetic amorphous silica powder of the present invention containsnumerous particles having been melted and spheroidized as shown in FIG.1 or FIG. 2, by cleaning and drying the powder after applying thespheroidizing treatment thereto. In this way, the synthetic amorphoussilica powder of the present invention contains numerous particlessubstantially resembling true spheres, respectively, so that thecircularity and unmolten ratio of the powder exhibit the above ranges,respectively, thereby decreasing the inevitable gas adsorption amount.It is noted that although melted and spheroidized particles occasionallycontain such particles each comprising agglomerated multiple particles,the above ratios such as the circularity and unmolten ratio are definedby assuming that the agglomerates each constitute a single particle.

Next will be explained a method for producing the synthetic amorphoussilica powder of the present invention. The method for producing asynthetic amorphous silica powder according to a first embodiment of thepresent invention is conducted as shown in FIG. 3, by applying aspheroidizing treatment to a silica powder to be used as a raw material,and by subsequently cleaning and drying it. Further, the method forproducing a synthetic amorphous silica powder according to a secondembodiment of the present invention is conducted as shown in FIG. 4, byfiring a silica powder to be used as a raw material, applying aspheroidizing treatment to the fired silica powder, and subsequentlycleaning and drying it. The respective steps will be explained in detailhereinafter.

The silica powder to be used as a raw material of the syntheticamorphous silica powder of the present invention is obtainable by thefollowing techniques, for example. As a first technique, ultrapure waterin an amount equivalent to 45 to 80 mols is firstly prepared per 1 molof silicon tetrachloride. The prepared ultrapure water is charged into avessel, and then the carbon tetrachloride is added thereinto, withstirring while keeping the temperature at 20 to 45° C. in an atmosphereof nitrogen, argon, or the like, thereby hydrolyzing the silicontetrachloride. After addition of the silicon tetrachloride, stirring iscontinued for 0.5 to 6 hours, thereby producing a siliceous gel. At thistime, it is preferable to set the stirring speed within a range of 100to 300 rpm. Next, the siliceous gel is transferred into a container fordrying which is brought into a drier, and the siliceous gel is dried for12 to 48 hours at a temperature of 200° C. to 300° C. while flowingnitrogen, argon, or the like through within the drier preferably at aflow rate of 10 to 20 L/min, thereby obtaining a dry powder. This drypowder is then taken out of the drier, and pulverized by a crusher suchas a roll crusher. In case of adopting a roll crusher, pulverizing isconducted by appropriately adjusting a roll gap to 0.2 to 2.0 mm and aroll revolution speed to 3 to 200 rpm. Finally, the pulverized particlesof the dry powder are classified by using a vibrating screen or thelike, thereby obtaining a silica powder having an average particlediameter D₅₀ of 10 to 3,000 μm, preferably 70 to 1,300 μm.

As a second technique, 0.5 to 3 mols of ultrapure water and 0.5 to 3mols of ethanol are prepared per 1 mol of tetramethoxysilane as anorganic silicon compound. The prepared ultrapure water and ethanol arecharged into a vessel, and then the tetramethoxysilane is addedthereinto, with stirring while keeping the temperature at 60° C. in anatmosphere of nitrogen, argon, or the like, thereby hydrolyzing thetetramethoxysilane. After addition of the tetramethoxysilane, stirringis continued for 5 to 120 minutes, and 1 to 50 mols of ultrapure wateris further added thereinto per 1 mol of tetramethoxysilane, followed bycontinued stirring for 1 to 12 hours, thereby producing a siliceous gel.At this time, it is preferable to set the stirring speed within a rangeof 100 to 300 rpm. Next, the siliceous gel is transferred into acontainer for drying which is brought into a drier, and the siliceousgel is dried for 6 to 48 hours at a temperature of 200° C. to 300° C.while flowing nitrogen, argon, or the like through within the drierpreferably at a flow rate of 10 to 20 L/min, thereby obtaining a drypowder. This dry powder is then taken out of the drier, and pulverizedby a crusher such as a roll crusher. In case of adopting a roll crusher,pulverizing is conducted by appropriately adjusting a roll gap to 0.2 to2.0 mm and a roll revolution speed to 3 to 200 rpm. Finally, thepulverized particles of the dry powder are classified by using avibrating screen or the like, thereby obtaining a silica powder havingan average particle diameter D₅₀ of 10 to 3,000 μm, preferably 70 to1,300 μm.

As a third technique, 3.0 to 35.0 mols of ultrapure water is firstlyprepared per 1 mol of fumed silica having an average particle diameterD₅₀ of 0.007 to 0.030 μm and a specific surface area of 50 to 380 m²/g.The prepared ultrapure water is charged into a vessel, and then thefumed silica is added thereinto, with stirring while keeping thetemperature at 10 to 30° C. in an atmosphere of nitrogen, argon, or thelike. After addition of the fumed silica, stirring is continued for 0.5to 6 hours, thereby producing a siliceous gel. At this time, it ispreferable to set the stirring speed within a range of 10 to 50 rpm.Next, the siliceous gel is transferred into a container for drying whichis brought into a drier, and the siliceous gel is dried for 12 to 48hours at a temperature of 200° C. to 300° C. while flowing nitrogen,argon, or the like through within the drier preferably at a flow rate of1 to 20 L/min, thereby obtaining a dry powder. This dry powder is thentaken out of the drier, and pulverized by a crusher such as a rollcrusher. In case of adopting a roll crusher, pulverizing is conducted byappropriately adjusting a roll gap to 0.2 to 2.0 mm and a rollrevolution speed to 3 to 200 rpm. Finally, the pulverized particles ofthe dry powder are classified by using a vibrating screen or the like,thereby obtaining a silica powder having an average particle diameterD₅₀ of 10 to 3,000 μm, preferably 70 to 1,300 μm.

While the thus obtained silica powder by granulation in the above manneris to be subjected to a spheroidizing treatment under a condition to bedescribed later, firing is to be conducted under a predeterminedcondition as shown in FIG. 4 before the spheroidizing treatment in caseof a method for producing a synthetic amorphous silica powder accordingto a second embodiment of the present invention. This firing isconducted in a vessel made of heat-resistant glass, quartz, or the like,at a temperature of 800 to 1,450° C. in an atmospheric air or in anitrogen atmosphere. By providing a firing step before a spheroidizingtreatment step, it becomes possible to remarkably decrease amounts ofgas components to be otherwise adsorbed on surfaces of powder particlesand gas components to be otherwise present within the powder particles.Further, since particles of the powder granulated from a fumed silicahave nano-sized closed pores therein, voids are left within theparticles when the powder is subjected to the spheroidizing treatment.Thus, it is possible to eliminate the nano-sized closed pores, by firingthe powder granulated from the fumed silica before the spheroidizingtreatment. Firing temperatures lower than the lower limit temperaturefail to sufficiently obtain such effects by virtue of firing, fordecreasing amounts of gas components, and for eliminating closed poresin the fumed silica. In turn, firing temperatures exceeding the upperlimit temperature lead to occurrence of such a problem that powderparticles are bound to one another.

The spheroidization of the silica powder obtained by any one of thefirst to third techniques, or the spheroidization of the silica powderobtained by firing the above obtained silica powder under the abovecondition, is attained by a spheroidizing treatment based on thermalplasma. In the spheroidizing treatment based on thermal plasma, it ispossible to use an apparatus shown in FIG. 5, for example. Thisapparatus 30 comprises a plasma torch 31 for generating plasma, achamber 32 as a reaction tube provided at a lower portion of the plasmatorch 31, and a collecting portion 33 provided at a lower portion of thechamber 32 so as to collect a powder after treatment. The plasma torch31 has: a quartz tube 34 communicated with the chamber 32 and sealed ata top portion; and a high-frequency inductive coil 36 wound around thequartz tube 34. The quartz tube 34 has an upper portion, which isprovided with a raw material supplying tube 37 therethrough, andconnected with a gas introducing tube 38. The chamber 32 is provided atits lateral side with a gas exhaust port 39. In the plasma torch 31,energization of the high-frequency inductive coil 36 generates a plasma40, and the quartz tube 34 is supplied with a gas such as argon, oxygen,or the like from the gas introducing tube 38. Further, supplied into theplasma 40 is a raw powder through the raw material supplying tube 37.The gas within the chamber 32 is exhausted from the gas exhaust port 39provided at the lateral side of the chamber 32.

In the first embodiment, argon as a working gas is introduced from thegas introducing tube 38 of the apparatus 30 at a flow rate of 60 to 100L/min, while applying a high frequency wave at a frequency of 2 to 3 MHzand at a power of 90 kW or higher, preferably 120 to 1,000 kW to theplasma torch 31, thereby generating a plasma. After the plasma isstabilized, oxygen is gradually introduced at a flow rate of 30 to 125L/min, thereby causing generation of an argon-oxygen plasma. Then, thesilica powder obtained by any one of the first to third techniques isdelivered from the raw material supplying tube 37 into the argon-oxygenplasma at a supplying rate of 3.5 to 17.5 kg/hr to thereby melt thesilica powder, such that particles now made into melted bodies arecaused to fall and collected by the collecting portion 33, therebyenabling to obtain spheroidized silica powder particles 41.

In the second embodiment, argon as a working gas is introduced from thegas introducing tube 38 of the apparatus 30 at a flow rate of 50 to 80L/min, while applying a high frequency wave at a frequency of 2 to 3 MHzand at a power of 90 to 120 kW to the plasma torch 31, therebygenerating a plasma. After the plasma is stabilized, oxygen is graduallyintroduced at a flow rate of 30 to 90 L/min, thereby causing generationof an argon-oxygen plasma. Then, the silica powder after firing isdelivered from the raw material supplying tube 37 into the argon-oxygenplasma at a supplying rate of 3.5 to 11.5 kg/hr to thereby melt thesilica powder, such that particles now made into melted bodies arecaused to fall and collected by the collecting portion 33, therebyenabling to obtain spheroidized silica powder particles 41.

Adjustments of a circularity, an unmolten ratio, and the like of thesynthetic amorphous silica powder can be conducted by adjusting thehigh-frequency power, the supplying rate of the raw silica powder, andthe like. For example, when the high-frequency power (W) is representedby A and the supplying rate (kg/hr) of the silica powder is representedby B within the above ranges, respectively, it is possible to obtaindesired circularity and unmolten ratio by adjusting the high-frequencypower A and the supplying rate B such that the value of A/B (W·hr/kg) isin a range of 1.0×10⁴ or more, under a condition that the high-frequencypower A is 90 kW or higher.

Because silica powder particles after the spheroidizing treatment havesurfaces carrying those fine particles attached thereto which have onceevaporated into the argon-oxygen plasma, ultrasonic cleaning isconducted such that the spheroidized silica powder particles after thespheroidizing step and ultrapure water are put into a cleaning vessel.Since the fine particles are migrated into the ultrapure water after theultrasonic cleaning, filtration is conducted by a filter having a coarsemesh. This operation is conducted repetitively until fine particles ofthe silica powder are fully filtered out.

For drying the silica powder after the cleaning step, the powder isfirstly put into a container for drying, and then the container fordrying is brought into a drier. Then, drying is preferably conducted byflowing nitrogen, argon, or the like at a flow rate of 1 to 20 L/minthrough within the drier, and by holding the powder at a temperature of100° C. to 400° C. for 3 to 48 hours.

By the above steps, the synthetic amorphous silica powder of the presentinvention is obtained. This synthetic amorphous silica powder is less ininevitable gas adsorption amount, so that the powder is preferablyusable as a raw material of a synthetic silica glass product.Particularly, according to the production method according to the secondembodiment of the present invention, the firing step under thepredetermined condition is provided before the spheroidizing treatmentstep, thereby enabling to remarkably decrease amounts of gas componentsto be otherwise adsorbed on surfaces of powder particles and gascomponents to be otherwise present within the powder particles.

EXAMPLES

Next, Examples of the present invention will be explained in detail,together with Comparative Examples.

Example 1

Firstly, ultrapure water was prepared in an amount equivalent to 55.6mols, per 1 mol of silicon tetrachloride. This ultrapure water wasbrought into a vessel, and then the carbon tetrachloride was addedthereinto, with stirring while keeping the temperature at 25° C. in anatmosphere of nitrogen, thereby hydrolyzing the silicon tetrachloride.After addition of the silicon tetrachloride, stirring was continued for2 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 100 rpm. Next, the siliceous gel was transferredinto a container for drying which was brought into a drier, and thesiliceous gel was dried for 24 hours at a temperature of 200° C. whileflowing nitrogen through within the drier at a flow rate of 10 L/min,thereby obtaining a dry powder. This dry powder was then taken out ofthe drier, and pulverized by a roll crusher. At this time, pulverizingwas conducted by adjusting a roll gap to 0.2 mm and a roll revolutionspeed to 40 rpm. The pulverized particles of the dry powder wereclassified by using a vibrating screen having openings of 100 μm and avibrating screen having openings of 154 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 124 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the cleaned powder was charged into a container for drying,then the container for drying was brought into a drier, and drying wasconducted by flowing nitrogen at a flow rate of 10 L/min through withinthe drier, and by holding the powder at a temperature of 200° C. for 48hours, thereby obtaining a synthetic amorphous silica powder.

Example 2

Firstly, 2 mols of ultrapure water and 2 mols of ethanol were preparedper 1 mol of tetramethoxysilane. The prepared ultrapure water andethanol were charged into a vessel, and then the tetramethoxysilane wasadded thereinto, with stirring while keeping the temperature at 60° C.in an atmosphere of nitrogen, thereby hydrolyzing thetetramethoxysilane. After addition of the tetramethoxysilane, stirringwas continued for 60 minutes, and 30 mols of ultrapure water was furtheradded thereinto per 1 mol of tetramethoxysilane, followed by continuedstirring for 5 hours, thereby producing a siliceous gel. At this time,the stirring speed was set to be 100 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 48 hours at a temperature of 200° C.while flowing nitrogen through within the drier at a flow rate of 20L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.2 mm and a rollrevolution speed to 55 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 100 μmand a vibrating screen having openings of 150 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 135 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of300° C. for 12 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 3

Firstly, 10 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.030 μm and aspecific surface area of 50 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 25° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 3 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 30 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 12 hours at a temperature of 300° C.while flowing nitrogen through within the drier at a flow rate of 10L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.5 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 400 μmand a vibrating screen having openings of 500 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 459 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 200 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 20 L/minthrough within the drier, and by holding the powder at a temperature of200° C. for 36 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 4

Conducted was the same procedure as Example 1 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below,and the obtained silica powder had an average particle diameter D₅₀ of900 μm.

Example 5

Firstly, 12 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.020 μm and aspecific surface area of 90 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 30° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 2 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 20 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 15 hours at a temperature of 250° C.while flowing nitrogen through within the drier at a flow rate of 1.0L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.2 mm and a rollrevolution speed to 25 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 50 μm anda vibrating screen having openings of 150 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 95 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces Of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of250° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 6

Firstly, ultrapure water was prepared in an amount equivalent to 60mols, per 1 mol of silicon tetrachloride. This ultrapure water wasbrought into a vessel, and then the carbon tetrachloride was addedthereinto, with stirring while keeping the temperature at 30° C. in anatmosphere of nitrogen, thereby hydrolyzing the silicon tetrachloride.After addition of the silicon tetrachloride, stirring was continued for4 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 250 rpm. Next, the siliceous gel was transferredinto a container for drying which was brought into a drier, and thesiliceous gel was dried for 24 hours at a temperature of 250° C. whileflowing nitrogen through within the drier at a flow rate of 10 L/min,thereby obtaining a dry powder. This dry powder was then taken out ofthe drier, and pulverized by a roll crusher. At this time, pulverizingwas conducted by adjusting a roll gap to 0.2 mm and a roll revolutionspeed to 150 rpm. The pulverized particles of the dry powder wereclassified by using a vibrating screen having openings of 50 μm and avibrating screen having openings of 200 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 116 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 15 L/min through within the drier,and by holding the powder at a temperature of 150° C. for 48 hours,thereby obtaining a synthetic amorphous silica powder.

Example 7

Firstly, 1 mol of ultrapure water and 1 mol of ethanol were prepared per1 mol of tetramethoxysilane. The prepared ultrapure water and ethanolwere charged into a vessel, and then the tetramethoxysilane was addedthereinto, with stirring while keeping the temperature at 60° C. in anatmosphere of nitrogen, thereby hydrolyzing the tetramethoxysilane.After addition of the tetramethoxysilane, stirring was continued for 60minutes, and 25 mols of ultrapure water was further added thereinto per1 mol of tetramethoxysilane, followed by continued stirring for 6 hours,thereby producing a siliceous gel. At this time, the stirring speed wasset to be 100 rpm. Next, the siliceous gel was transferred into acontainer for drying which was brought into a drier, and the siliceousgel was dried for 48 hours at a temperature of 150° C. while flowingnitrogen through within the drier at a flow rate of 20 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 0.2 mm and a roll revolution speedto 55 rpm. The pulverized particles of the dry powder were classified byusing a vibrating screen having openings of 50 μm and a vibrating screenhaving openings of 200 μm, thereby obtaining a silica powder having anaverage particle diameter D₅₀ of 118 μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment to the above obtained silica powder under anapplicable condition shown in Table 1 described below, withoutconducting firing of the silica powder. Specifically, argon as a workinggas was introduced from the gas introducing tube 38 of the apparatus 30,and a high frequency wave was applied to the plasma torch 31 to generatea plasma. After the plasma was stabilized, oxygen was graduallyintroduced, thereby causing generation of an argon-oxygen plasma. Then,the above obtained silica powder was delivered from the raw materialsupplying tube 37 into the argon-oxygen plasma to thereby melt thesilica powder, such that particles now made into melted bodies werecaused to fall and collected by the collecting portion 33, therebyobtaining spheroidized silica powder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 15 L/min through within the drier,and by holding the powder at a temperature of 150° C. for 48 hours,thereby obtaining a synthetic amorphous silica powder.

Comparative Example 1

Conducted was the same procedure as Example 1 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below.

Comparative Example 2

Conducted was the same procedure as Example 2 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below.

Comparative Example 3

Conducted was the same procedure as Example 3 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below.

Comparative Example 4

Conducted was the same procedure as Example 3 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below.

Comparative Example 5

Conducted was the same procedure as Example 4 to obtain a syntheticamorphous silica powder, except that the spheroidizing treatment wasapplied under an applicable condition shown in Table 1 described below.

Comparative Example 6

Firstly, ultrapure water was prepared in an amount equivalent to 55.6mols, per 1 mol of silicon tetrachloride. This ultrapure water wasbrought into a vessel, and then the carbon tetrachloride was addedthereinto, with stirring while keeping the temperature at 25° C. in anatmosphere of nitrogen, thereby hydrolyzing the silicon tetrachloride.After addition of the silicon tetrachloride, stirring was continued for2 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 100 rpm. Next, the siliceous gel was transferredinto a container for drying which was brought into a drier, and thesiliceous gel was dried for 24 hours at a temperature of 200° C. whileflowing nitrogen through within the drier at a flow rate of 10 L/min,thereby obtaining a dry powder. This dry powder was then taken out ofthe drier, and pulverized by a roll crusher. At this time, pulverizingwas conducted by adjusting a roll gap to 0.2 mm and a roll revolutionspeed to 40 rpm. The pulverized particles of the dry powder wereclassified by using a vibrating screen having openings of 100 μm and avibrating screen having openings of 150 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 128 μm.

Finally, the pulverized powder was charged into a vessel for firing,then the vessel for firing was brought into a firing furnace, and firingwas conducted by flowing nitrogen at a flow rate of 10 L/min throughwithin the firing furnace, and by holding the powder at a temperature of1,200° C. for 48 hours, thereby obtaining a synthetic amorphous silicapowder having an average particle diameter D₅₀ of 91 μm. This silicapowder without applying a spheroidizing treatment thereto, was made tobe Comparative Example 6.

Comparative Example 7

Firstly, 2 mols of ultrapure water and 2 mols of ethanol were preparedper 1 mol of tetramethoxysilane. The prepared ultrapure water andethanol were charged into a vessel, and then the tetramethoxysilane wasadded thereinto, with stirring while keeping the temperature at 60° C.in an atmosphere of nitrogen, thereby hydrolyzing thetetramethoxysilane. After addition of the tetramethoxysilane, stirringwas continued for 60 minutes, and 30 mols of ultrapure water was furtheradded thereinto per 1 mol of tetramethoxysilane, followed by continuedstirring for 6 hours, thereby producing a siliceous gel. At this time,the stirring speed was set to be 100 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 48 hours at a temperature of 200° C.while flowing nitrogen through within the drier at a flow rate of 20L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.4 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 400 μmand a vibrating screen having openings of 500 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 469 μm.

Finally, the pulverized powder was charged into a vessel for firing,then the vessel for firing was brought into a firing furnace, and firingwas conducted by flowing nitrogen at a flow rate of 10 L/min throughwithin the firing furnace, and by holding the powder at a temperature of1,200° C. for 48 hours, thereby obtaining a synthetic amorphous silicapowder having an average particle diameter D₅₀ of 328 μm. This silicapowder without applying a spheroidizing treatment thereto, was made tobe Comparative Example 7.

Comparative Example 8

Firstly, 10 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.030 μm and aspecific surface area of 50 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 25° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 3 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 30 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 12 hours at a temperature of 300° C.while flowing nitrogen through within the drier at a flow rate of 10L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.9 mm and a rollrevolution speed to 150 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 800 μmand a vibrating screen having openings of 900 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 849 μm.

Finally, the pulverized powder was charged into a vessel for firing,then the vessel for firing was brought into a firing furnace, and firingwas conducted by flowing nitrogen at a flow rate of 10 L/min throughwithin the firing furnace, and by holding the powder at a temperature of1,200° C. for 48 hours, thereby obtaining a synthetic amorphous silicapowder having an average particle diameter D₅₀ of 594 μm. This silicapowder without applying a spheroidizing treatment thereto, was made tobe Comparative Example 8.

TABLE 1 Silica powder Spheroidizing treatment condition Average parti-Frequency cle diameter of high- High- Ar gas Oxygen Raw powder D50[μm]frequency frequency flow flow supplying Raw Before After wave power Arate rate rate A/B material firing firing [MHz] [kW] [L/min] [L/min]B[kg/hr] [W · hr/kg] Example 1 Silicon 124 — 3 90 60 30 3.9 2.3 × 10⁴tetrachloride Example 2 Tetrameth- 135 — 3 90 50 50 6.4 1.4 × 10⁴oxysilane Example 3 Fumed 459 — 2 120 80 60 9.8 1.2 × 10⁴ silica Example4 Silicon 900 — 2 180 100 125 17.1 1.1 × 10⁴ tetrachloride Example 5Fumed 95 — 2 120 80 65 9.6 1.3 × 10⁴ silica Example 6 Silicon 116 — 1120 35 150 8.2 1.5 × 10⁴ tetrachloride Example 7 Tetrameth- 118 — 1 12040 155 8.3 1.4 × 10⁴ oxysilane Comparative Silicon 124 — 2 120 40 14013.3 9.0 × 10³ Example 1 tetrachloride Comparative Tetrameth- 136 — 1120 30 160 12.2 9.8 × 10³ Example 2 oxysilane Comparative Fumed 459 — 190 20 125 9.3 9.7 × 10³ Example 3 silica Comparative Fumed 459 — 1 60 20135 6.2 9.7 × 10³ Example 4 silica Comparative Silicon 900 — 1 30 5 506.1 4.9 × 10³ Example 5 tetrachloride Comparative Silicon 128 — — — — —— — Example 6 tetrachloride Comparative Tetrameth- 469 — — — — — — —Example 7 oxysilane Comparative Fumed 849 — — — — — — — Example 8 silica

Example 8

Firstly, 12 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.020 μm and aspecific surface area of 90 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 30° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 2 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 20 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 15 hours at a temperature of 250° C.while flowing nitrogen through within the drier at a flow rate of 10L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.2 mm and a rollrevolution speed to 25 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 75 μm anda vibrating screen having openings of 200 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 140 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,250° C. for 48 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 97μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing argon at a flow rate of 10 L/min throughwithin the drier, and by holding the powder at a temperature of 250° C.for 24 hours, thereby obtaining a synthetic amorphous silica powder.

Example 9

Firstly, 5 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.030 μm and aspecific surface area of 50 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 20° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 0.5 hour, thereby producing a siliceous gel. At this time, thestirring speed was set to be 30 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 48 hours at a temperature of 200° C.while flowing nitrogen through within the drier at a flow rate of 15L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.3 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 75 μm anda vibrating screen having openings of 250 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 155 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,300° C. for 24 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 105μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 1 L/minthrough within the drier, and by holding the powder at a temperature of400° C. for 12 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 10

Firstly, 30 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.007 μm and aspecific surface area of 300 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 10° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 6 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 50 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 12 hours at a temperature of 300° C.while flowing nitrogen through within the drier at a flow rate of 15L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.5 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 300 μmand a vibrating screen having openings of 600 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 482 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,350° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 342μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 200 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of250° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 11

Firstly, 15 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.016 μm and aspecific surface area of 130 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 25° C. in an atmosphereof argon. After addition of the fumed silica, stirring was continued for3 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 15 rpm. Next, the siliceous gel was transferred intoa container for drying which was brought into a drier, and the siliceousgel was dried for 36 hours at a temperature of 200° C. while flowingnitrogen through within the drier at a flow rate of 10 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 1.0 mm and a roll revolution speedto 50 rpm. The pulverized particles of the dry powder were classified byusing a vibrating screen having openings of 500 μm and a vibratingscreen having openings of 1,500 μm, thereby obtaining a silica powderhaving an average particle diameter D₅₀ of 1,083 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,450° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 725μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 400 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of300° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Example 12

Firstly, ultrapure water was prepared in an amount equivalent to 60mols, per 1 mol of silicon tetrachloride. This ultrapure water wasbrought into a vessel, and then the silicon tetrachloride was addedthereinto, with stirring while keeping the temperature at 30° C. in anatmosphere of nitrogen, thereby hydrolyzing the silicon tetrachloride.After addition of the silicon tetrachloride, stirring was continued for4 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 250 rpm. Next, the siliceous gel was transferredinto a container for drying which was brought into a drier, and thesiliceous gel was dried for 24 hours at a temperature of 250° C. whileflowing nitrogen through within the drier at a flow rate of 10 L/min,thereby obtaining a dry powder. This dry powder was then taken out ofthe drier, and pulverized by a roll crusher. At this time, pulverizingwas conducted by adjusting a roll gap to 0.2 mm and a roll revolutionspeed to 150 rpm. The pulverized particles of the dry powder wereclassified by using a vibrating screen having openings of 50 μm and avibrating screen having openings of 200 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 151 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,400° C. for 36 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 111μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 15 L/min through within the drier,and by holding the powder at a temperature of 150° C. for 48 hours,thereby obtaining a synthetic amorphous silica powder.

Example 13

Firstly, 1 mol of ultrapure water and 1 mol of ethanol were prepared per1 mol of tetramethoxysilane. The prepared ultrapure water and ethanolwere charged into a vessel, and then the tetramethoxysilane was addedthereinto, with stirring while keeping the temperature at 60° C. in anatmosphere of nitrogen, thereby hydrolyzing the tetramethoxysilane.After addition of the tetramethoxysilane, stirring was continued for 60minutes, and 25 mols of ultrapure water was further added thereinto per1 mol of tetramethoxysilane, followed by continued stirring for 6 hours,thereby producing a siliceous gel. At this time, the stirring speed wasset to be 100 rpm. Next, the siliceous gel was transferred into acontainer for drying which was brought into a drier, and the siliceousgel was dried for 24 hours at a temperature of 200° C. while flowingnitrogen through within the drier at a flow rate of 20 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 0.2 mm and a roll revolution speedto 55 rpm. The pulverized particles of the dry powder were classified byusing a vibrating screen having openings of 75 μm and a vibrating screenhaving openings of 250 μm, thereby obtaining a silica powder having anaverage particle diameter D₅₀ of 147 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,200° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 107μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 10 L/min through within the drier,and by holding the powder at a temperature of 300° C. for 12 hours,thereby obtaining a synthetic amorphous silica powder.

Comparative Example 9

Firstly, 12 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.020 μm and aspecific surface area of 90 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 30° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 2 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 20 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 15 hours at a temperature of 250° C.while flowing nitrogen through within the drier at a flow rate of 10L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.2 mm and a rollrevolution speed to 25 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 75 μm anda vibrating screen having openings of 250 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 144 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,200° C. for 36 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 101μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of250° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Comparative Example 10

Firstly, 5 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.030 μm and aspecific surface area of 50 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 20° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 0.5 hour, thereby producing a siliceous gel. At this time, thestirring speed was set to be 30 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 48 hours at a temperature of 200° C.while flowing nitrogen through within the drier at a flow rate of 15L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.3 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 75 μm anda vibrating screen having openings of 250 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 174 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,250° C. for 24 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 115μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 1 L/minthrough within the drier, and by holding the powder at a temperature of400° C. for 12 hours, thereby obtaining a synthetic amorphous silicapowder.

Comparative Example 11

Firstly, 30 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.007 μm and aspecific surface area of 300 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 10° C. in an atmosphereof nitrogen. After addition of the fumed silica, stirring was continuedfor 6 hours, thereby producing a siliceous gel. At this time, thestirring speed was set to be 50 rpm. Next, the siliceous gel wastransferred into a container for drying which was brought into a drier,and the siliceous gel was dried for 12 hours at a temperature of 300° C.while flowing nitrogen through within the drier at a flow rate of 15L/min, thereby obtaining a dry powder. This dry powder was then takenout of the drier, and pulverized by a roll crusher. At this time,pulverizing was conducted by adjusting a roll gap to 0.5 mm and a rollrevolution speed to 100 rpm. The pulverized particles of the dry powderwere classified by using a vibrating screen having openings of 300 μmand a vibrating screen having openings of 700 μm, thereby obtaining asilica powder having an average particle diameter D₅₀ of 516 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,350° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 361μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 200 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of250° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Comparative Example 12

Firstly, 15 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.016 μm and aspecific surface area of 130 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 25° C. in an atmosphereof argon. After addition of the fumed silica, stirring was continued for4 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 15 rpm. Next, the siliceous gel was transferred intoa container for drying which was brought into a drier, and the siliceousgel was dried for 36 hours at a temperature of 200° C. while flowingargon through within the drier at a flow rate of 10 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 1.0 mm and a roll revolution speedto 50 rpm. The pulverized particles of the dry powder were classified byusing a vibrating screen having openings of 500 μm and a vibratingscreen having openings of 1,500 μm, thereby obtaining a silica powderhaving an average particle diameter D₅₀ of 1,030 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,450° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 711μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 400 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of300° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Comparative Example 13

Firstly, 15 mols of ultrapure water was prepared per 1 mol of fumedsilica having an average particle diameter D₅₀ of 0.016 μm and aspecific surface area of 130 m²/g. The prepared ultrapure water wascharged into a vessel, and then the fumed silica was added thereinto,with stirring while keeping the temperature at 25° C. in an atmosphereof argon. After addition of the fumed silica, stirring was continued for3 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 15 rpm. Next, the siliceous gel was transferred intoa container for drying which was brought into a drier, and the siliceousgel was dried for 36 hours at a temperature of 200° C. while flowingnitrogen through within the drier at a flow rate of 10 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 1.0 mm and a roll revolution speedto 40 rpm. The pulverized particles of the dry powder were classified byusing a vibrating screen having openings of 500 μm and a vibratingscreen having openings of 1,500 μm, thereby obtaining a silica powderhaving an average particle diameter D₅₀ of 1,030 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,450° C. for 72 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 711μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 400 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, then the container for drying was brought into a drier, anddrying was conducted by flowing nitrogen at a flow rate of 10 L/minthrough within the drier, and by holding the powder at a temperature of300° C. for 24 hours, thereby obtaining a synthetic amorphous silicapowder.

Comparative Example 14

Firstly, ultrapure water was prepared in an amount equivalent to 60mols, per 1 mol of silicon tetrachloride. This ultrapure water wasbrought into a vessel, and then the carbon tetrachloride was addedthereinto, with stirring while keeping the temperature at 30° C. in anatmosphere of nitrogen, thereby hydrolyzing the silicon tetrachloride.After addition of the silicon tetrachloride, stirring was continued for4 hours, thereby producing a siliceous gel. At this time, the stirringspeed was set to be 250 rpm. Next, the siliceous gel was transferredinto a container for drying which was brought into a drier, and thesiliceous gel was dried for 24 hours at a temperature of 250° C. whileflowing nitrogen through within the drier at a flow rate of 10 L/min,thereby obtaining a dry powder. This dry powder was then taken out ofthe drier, and pulverized by a roll crusher. At this time, pulverizingwas conducted by adjusting a roll gap to 0.2 mm and a roll revolutionspeed to 150 rpm. The pulverized particles of the dry powder wereclassified by using a vibrating screen having openings of 50 μm and avibrating screen having openings of 200 μm, thereby obtaining a silicapowder having an average particle diameter D₅₀ of 165 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,350° C. for 24 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 112μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 15 L/min through within the drier,and by holding the powder at a temperature of 150° C. for 48 hours,thereby obtaining a synthetic amorphous silica powder.

Comparative Example 15

Firstly, 1 mol of ultrapure water and 1 mol of ethanol were prepared per1 mol of tetramethoxysilane. The prepared ultrapure water and ethanolwere charged into a vessel, and then the tetramethoxysilane was addedthereinto, with stirring while keeping the temperature at 60° C. in anatmosphere of nitrogen, thereby hydrolyzing the tetramethoxysilane.After addition of the tetramethoxysilane, stirring was continued for 60minutes, and 25 mols of ultrapure water was further added thereinto per1 mol of tetramethoxysilane, followed by continued stirring for 6 hours,thereby producing a siliceous gel. At this time, the stirring speed wasset to be 100 rpm. Next, the siliceous gel was transferred into acontainer for drying which was brought into a drier, and the siliceousgel was dried for 24 hours at a temperature of 200° C. while flowingnitrogen through within the drier at a flow rate of 20 L/min, therebyobtaining a dry powder. This dry powder was then taken out of the drier,and pulverized by a roll crusher. At this time, pulverizing wasconducted by adjusting a roll gap to 0.2 mm and a roll revolution speedto 150 rpm. The pulverized particles of the dry powder were classifiedby using a vibrating screen having openings of 50 μm and a vibratingscreen having openings of 200 μm, thereby obtaining a silica powderhaving an average particle diameter D₅₀ of 159 μm.

The granulated powder was put into a quartz vessel and subjected tofiring in the atmospheric air at 1,350° C. for 24 hours, therebyobtaining a silica powder having an average particle diameter D₅₀ of 111μm.

Subsequently, the apparatus 30 shown in FIG. 5 was used to apply aspheroidizing treatment, under an applicable condition shown in Table 2described below, to the silica powder obtained after firing.Specifically, argon as a working gas was introduced from the gasintroducing tube 38 of the apparatus 30, and a high frequency wave wasapplied to the plasma torch 31 to generate a plasma. After the plasmawas stabilized, oxygen was gradually introduced, thereby causinggeneration of an argon-oxygen plasma. Then, the above obtained silicapowder was delivered from the raw material supplying tube 37 into theargon-oxygen plasma to thereby melt the silica powder, such thatparticles now made into melted bodies were caused to fall and collectedby the collecting portion 33, thereby obtaining spheroidized silicapowder particles 41.

After the spheroidizing treatment, the powder and ultrapure water wereput into a cleaning vessel, to conduct ultrasonic cleaning. Afterconducting the ultrasonic cleaning, filtration was conducted by a filterhaving openings of 50 μm. This operation was conducted repetitivelyuntil fine particles attached to surfaces of the silica powder particleswere fully filtered out.

Finally, the powder after cleaning was charged into a container fordrying, which was then brought into a drier, and drying was conducted byflowing nitrogen at a flow rate of 15 L/min through within the drier,and by holding the powder at a temperature of 150° C. for 48 hours,thereby obtaining a synthetic amorphous silica powder.

TABLE 2 Silica powder Spheroidizing treatment condition Average parti-Frequency cle diameter of high- High- Ar gas Oxygen Raw powder D50[μm]frequency frequency flow flow supplying Raw Before After wave power Arate rate rate A/B material firing firing [MHz] [kW] [L/min] [L/min]B[kg/hr] [W · hr/kg] Example 8 Fumed 140 97 3 90 60 30 3.9 2.3 × 10⁴silica Example 9 Fumed 155 105 3 90 50 50 6.4 1.4 × 10⁴ silica Example10 Fumed 482 342 2 120 80 60 9.8 1.2 × 10⁴ silica Example 11 Fumed 1083725 2 180 100 125 17.1 1.1 × 10⁴ silica Example 12 Silicon 151 111 3 9045 45 6.2 1.5 × 10⁴ tetrachloride Example 13 Tetrameth- 147 107 3 90 4545 5.8 1.6 × 10⁴ oxysilane Comparative Fumed 144 101 1 120 30 160 12.29.8 × 10³ Example 9 silica Comparative Fumed 174 115 1 120 45 155 14.28.5 × 10³ Example 10 silica Comparative Fumed 516 361 1 60 40 80 8.3 7.2× 10³ Example 11 silica Comparative Fumed 1030 711 1 60 40 80 9.1 6.6 ×10³ Example 12 silica Comparative Fumed 1030 711 1 30 25 40 6.5 4.6 ×10³ Example 13 silica Comparative Silicon 165 112 1 120 35 150 12.4 9.7× 10³ Example 14 tetrachloride Comparative Tetrameth- 159 111 1 120 40155 12.8 9.4 × 10³ Example 15 oxysilane

Measured for the powders obtained in Examples 1 to 13 and ComparativeExamples 1 to 15, were an average particle diameter D₅₀, a BET specificsurface area, a theoretical specific surface area, a quotientrepresented by “BET specific surface area/theoretical specific surfacearea”, a real density, an intra-particulate porosity, a circularity, andan unmolten ratio, by those techniques to be described hereinafter.These results are listed in Table 3 or Table 4.

(1) Average particle diameter D₅₀: this was obtained by calculating anaverage value of medians of particle distributions (diameter) measuredthree times by Laser Diffraction/Scattering Particle Size DistributionAnalyzer (Model Name: HORIBA LA-950).

(2) BET specific surface area: this was measured by a BET three-pointmethod by using a measuring apparatus (QUANTACHROME AUTOSORB-1 MP). TheBET three-point method is configured to obtain a gradient A fromadsorbed nitrogen amounts at three points of relative pressure, therebyobtaining a specific surface area value based on a BET equation.Measurement of adsorbed nitrogen amounts was conducted under a conditionof 150° C. for 60 minutes.

(3) Theoretical specific surface area: this was calculated from thefollowing equation (2), assuming that D represents an average particlediameter D₅₀ of a powder, and ρ represents a real density of 2.2 g/cm³in the following equation (1):

Theoretical specific surface area=6/(D×ρ)  (1)

Theoretical specific surface area of powder=2.73/D ₅₀  (2)

(4) Quotient of “BET specific surface area/theoretical specific surfacearea”: this was calculated from the BET specific surface area and thetheoretical specific surface area measured and obtained in the abovemanners, respectively.

(5) Real density: this was calculated as an average value of absolutedensities obtained by conducting a real density measurement three timesin conformity to JIS R7212: Testing Methods for Carbon Blocks, (d)Measurement of true specific gravity.

(6) Intra-particulate porosity: the obtained powder was embedded in aresin which was then ground to expose cross sections of powderparticles. The cross sections of powder particles were observed by a SEM(scanning electron microscope). Further, the intra-particulate porositywas calculated from the following equation (4), by measuringcross-sectional areas of 50 powder particles, and areas of spaces in theparticles, if present:

Intra-particulate porosity=total area of spaces in particles/totalcross-sectional area of particles (4)

(7) Circularity: this was measured two times by a particle size/shapedistribution measuring device (PITA-1 manufactured by SEISHIN ENTERPRISECo., Ltd.) shown in FIG. 4, and average values thereof were calculated.Specifically, powder particles were firstly dispersed into a liquid,which was then flowed through a planar elongational flow cell 51.Recorded as images by an objective lens 53 were 200 powder particles 52moving through within the planar elongational flow cell 51,respectively, thereby calculating a circularity from the recorded imagesand the following equation (3). In the equation (3), S represents anarea of each recorded particle image in a projection view, and Lrepresents a perimeter of the particle image in the projection view. Thecircularity of the applicable powder was provided as an average value ofcircularities of 200 particles calculated in the above manner.

Circularity=4ηS/L ²  (3)

(8) Unmolten ratio: this was calculated as a ratio at which angulateparticles as shown in FIG. 6 were contained in 200 particles of thepowder in the above-mentioned projection view thereof.

<Comparative Test and Evaluation 1>

The powders obtained in Examples 1 to 13 and Comparative Examples 1 to15 were used to fabricate rectangular parallelepiped block materials oflength 20 mm×width 20 mm×height 40 mm, respectively, in a manner toevaluate the number of gas bubbles caused in each block material. Theseresults are listed in Table 3 or Table 4. Specifically, each blockmaterial was fabricated by: introducing the applicable powder into acarbon crucible; heating it by a carbon heater to 2,200° C. in a vacuumatmosphere at 2.0×10⁴ Pa; and holding it for 48 hours. Conducted forthis block material was a heat treatment at a temperature of 1, 600° C.for 48 hours in a vacuum atmosphere of 5.0×10² Pa. After the heattreatment, the block material was cut out at a height of 20 mm to exposea cross section of length 20 mm×width 20 mm which was then ground, in amanner to evaluate the number of gas bubbles observed in a region havinga depth of 2 mm from the surface (cross section) of the block material,and a width of 2 mm.

TABLE 3 Average BET Theoretical Quotient of BET particle specificspecific specific surface Number diameter surface surface area/theoret-Real Intra- of gas D50 area area ical specific density particulateCircu- Unmolton bubbles [μm] [m2/g] [m2/g] surface area [g/cm3] porositylarity ratio [count] Example 1 98 0.028 0.028 1.00 2.20 0.00 1.00 0.00 6Example 2 107 0.029 0.026 1.12 2.19 0.01 0.88 0.04 10 Example 3 3530.010 0.008 1.25 2.18 0.02 0.81 0.15 12 Example 4 711 0.005 0.004 1.252.12 0.04 0.75 0.25 18 Example 5 104 0.027 0.026 1.04 2.18 0.01 0.990.05 9 Example 6 128 0.025 0.021 1.19 2.18 0.02 0.96 0.09 11 Example 7132 0.025 0.021 1.19 2.17 0.02 0.96 0.1 12 Comparative 111 0.039 0.0251.56 2.17 0.04 0.85 0.08 39 Example 1 Comparative 118 0.030 0.023 1.302.04 0.05 0.81 0.15 63 Example 2 Comparative 382 0.009 0.007 1.29 2.140.07 0.79 0.18 68 Example 3 Comparative 402 0.009 0.007 1.29 2.12 0.030.67 0.20 72 Example 4 Comparative 766 0.005 0.004 1.25 2.11 0.04 0.770.34 83 Example 5 Comparative 91 0.110 0.030 3.67 2.19 0.01 — — 116Example 6 Comparative 328 0.023 0.008 2.88 2.20 0.00 — — 93 Example 7Comparative 594 0.022 0.005 4.40 2.18 0.03 — — 132 Example 8

As apparent from Table 3, it is seen that the blocks fabricated by usingthe powders of Examples 1 to 7, respectively, were remarkably decreasedin the number of caused gas bubbles, as compared to the blocksfabricated by using the powders of Comparative Examples 6 to 8 withoutsubjecting to a spheroidizing treatment, respectively. Further, it isseen that, comparing Examples 1 to 7 with Comparative Examples 1 to 5,Examples 1 to 7 were remarkably decreased in the number of caused gasbubbles as compared to Comparative Examples 1 to 5 though these Examplesand Comparative Examples were each subjected to a spheroidizingtreatment. Revealed from these results were a presence of an optimumcondition of the spheroidizing treatment, for allowing to decrease anamount of generation of gas bubbles.

TABLE 4 Average BET Theoretical Quotient of BET particle specificspecific specific surface Number diameter surface surface area/theoret-Real Intra- of gas D50 area area ical specific density particulateCircu- Unmolton bubbles [μm] [m2/g] [m2/g] surface area [g/cm3] porositylarity ratio [count] Example 8 106 0.026 0.026 1 2.2 0.01 1 0.02 0Example 9 110 0.027 0.025 1.08 2.2 0 0.97 0.05 0 Example 10 352 0.0090.008 1.13 2.17 0.03 0.87 0.11 1 Example 11 761 0.005 0.004 1.25 2.150.03 0.78 0.23 3 Example 12 118 0.024 0.023 1.04 2.2 0 0.98 0.04 2Example 13 117 0.025 0.023 1.09 2.2 0 0.97 0.04 1 Comparative 108 0.0350.025 1.4 2.13 0.01 0.81 0.2 7 Example 9 Comparative 121 0.028 0.0231.22 2.07 0.03 0.82 0.14 15 Example 10 Comparative 383 0.009 0.007 1.292.12 0.08 0.8 0.23 23 Example 11 Comparative 761 0.005 0.004 1.25 2.150.02 0.72 0.24 18 Example 12 Comparative 754 0.005 0.004 1.25 2.16 0.030.77 0.27 13 Example 13 Comparative 114 0.034 0.024 1.42 2.19 0.01 0.990.03 6 Example 14 Comparative 118 0.033 0.023 1.43 2.18 0.02 0.98 0.04 7Example 15

As apparent from Table 1 to Table 4, it is seen that the blocksfabricated by using the powders of Examples 8 to 13 which were firedunder predetermined conditions before spheroidizing treatments,respectively, were further decreased in the number of caused gasbubbles, as compared to the blocks fabricated by using the powders ofExamples 1 to 7 without subjecting to firing, respectively.

Further, as apparent from Table 4, and comparing Examples 8 to 11 withComparative Examples 9 to 13, Example 12 with Comparative Example 14,and Example 13 with Comparative Example 15, it is seen therefrom thatExamples 8 to 11, Example 12, and Example 13 were remarkably decreasedin the number of caused gas bubbles, as compared to Comparative Examples9 to 13, Comparative Example 14, and Comparative Example 15, thoughthese Examples and Comparative Examples were each subjected to aspheroidizing treatment. It is seen therefrom that the syntheticamorphous silica powders of the present invention are each remarkablyenhanced in effect for reducing an amount of generation or degree ofexpansion of gas bubbles, and are improved in formability, so that thepowders are each suitable as a raw material for manufacturing asynthetic silica glass product.

<Evaluation 2>

Impurity concentrations of the powders obtained in Examples 1 to 13 andComparative Examples 1 to 15 were analyzed or measured by the followingtechniques (1) to (5). The results thereof are listed in Table 5 orTable 6.

(1) Na, K, Ca, Fe, Al, and P: Each synthetic amorphous silica powder wasthermally decomposed with hydrofluoric acid and sulfuric acid, in amanner to prepare a constant-volume liquid by using dilute nitric acidafter thermal condensation, followed by conduction of analysis by aHigh-Frequency Inductive Coupling Plasma Mass Spectrometer (Model Name:SPQ9000 of SII NanoTechnology Inc.).

(2) B: Each synthetic amorphous silica powder was thermally decomposedwith hydrofluoric acid, in a manner to prepare a constant-volume liquidby using ultrapure water after thermal condensation, followed byconduction of analysis by a High-Frequency Inductive Coupling PlasmaMass Spectrometer (Model Name: SPQ9400 of SII NanoTechnology Inc.)

(3) C: Added to each synthetic amorphous silica powder were iron,tungsten, and tin as combustion improvers, to thereby conduct analysisby a high-frequency furnace combustion infrared absorption method (ModelName: HORIBA EMIA-920V).

(4) Cl: Each synthetic amorphous silica powder was mixed with ultrapurewater, in a manner to cause Cl to leach out of the former into thelatter, under ultrasonic waves. The synthetic amorphous silica powderand the leach solution were separated from each other by a centrifuge,and the separated leach solution was subjected to conduction of analysisby ion chromatography (Model Name: Dionex DX-500).

(5) OH: Measurement therefor was conducted by a peak height near 3,660cm⁻¹ by Fourier Transformation Infrared spectrophotometer (Model Name:ThermoFischer Nicolet 4700FT-IR).

TABLE 5 Unit: [mass ppm] Na K Ca Fe Al B C P Cl OH Example 1

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  3 41 Example 2

 0.01

 0.01 0.05 0.1 0.1

 0.01  4

 0.01

 1 48 Example 3

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  2 59 Example 4

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  4 52 Example 5

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  2 51 Example 6

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01  6 65 Example 7

 0.01

 0.01 0.05 0.1 0.1

 0.01  8

 0.01

 2 62 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  5 61 Example 1 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  9

 0.01

 1 60 Example 2 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  6 67 Example 3 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  6 72 Example 4 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  2

 0.01  7 78 Example 5 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  3

 0.01  8 98 Example 6 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01 11

 0.01

 1 102 Example 7 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  4

 0.01  6 91 Example 8

As apparent from Table 5, it is seen that Examples 1 to 7 were eachrelatively low in concentration of hydroxyl group and carbon whichpossibly act as gas components acting as sources of generation orexpansion of gas bubbles in a synthetic silica glass product at a hightemperature and a reduced pressure, as compared to Comparative Examples1 to 8.

TABLE 6 Unit: [mass ppm] Na K Ca Fe Al B C P Cl OH Example 8

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 28 Example 9

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 26 Example 10

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 25 Example 11

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 28 Example 12

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 48 Example 13

 0.01

 0.01 0.05 0.1 0.1

 0.01  3

 0.01  3 51 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 31 Example 9 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 35 Example 10 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 33 Example 11 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 35 Example 12 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01

 2 37 Example 13 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01

 2

 0.01  3 40 Example 14 Comparative

 0.01

 0.01 0.05 0.1 0.1

 0.01  4

 0.01

 2 47 Example 15

As apparent from Table 6, it is seen that: the synthetic amorphoussilica powders each adopting a powder as a raw material obtained byreacting silicon tetrachloride in a liquid, were less than 2 ppm incarbon concentration; the synthetic amorphous silica powders eachadopting a powder as a raw material obtained from an organic siliconcompound, were less than 2 ppm in chlorine concentration; and thesynthetic amorphous silica powders each adopting a powder as a rawmaterial obtained from fumed silica, were less than 2 ppm in carbonconcentration and less than 2 ppm in chlorine concentration.

INDUSTRIAL APPLICABILITY

The synthetic amorphous silica powder of the present invention ispreferably usable as a raw material for manufacturing a synthetic silicaglass product such as a crucible, jig, and the like to be used forsingle crystal production in semiconductor application.

1-9. (canceled)
 10. A method for producing a synthetic amorphous silicapowder, comprising, in the recited order: a granulating step forproducing a siliceous gel, drying the siliceous gel to turn it into adry powder, pulverizing particles of the dry powder, and thenclassifying the pulverizedly obtained particles to thereby obtain asilica powder; a firing step for firing particles of the silica powderobtained in the granulating step, at a temperature of 800 to 1,450° C.;a spheroidizing step based on a thermal plasma for delivering, at apredetermined supplying rate, particles of the silica powder obtained inthe firing step into a plasma torch in which a plasma is generated by apredetermined high-frequency power, in a manner to heat the particles ata temperature from 2,000° C. to a boiling point of silicon dioxide,thereby melting the particles; a cleaning step for removing fineparticles attached to surfaces of the spheroidized silica powderparticles after the spheroidizing step; and a drying step for drying thesilica powder particles after the cleaning step; wherein thespheroidizing step is conducted by adjusting a value of A/B (W·hr/kg) to1.0×10⁴ or more, where A is the high-frequency power (W), and B is thesupplying rate (kg/hr) of the silica powder, under a condition that thehigh-frequency power A is 90 kW or higher, thereby obtaining a syntheticamorphous silica powder having: an average particle diameter D₅₀ of 10to 2,000 μm; a quotient of 1.00 to 1.35 obtained by dividing a BETspecific surface area of the powder by a theoretical specific surfacearea calculated from the average particle diameter D₅₀; a real densityof 2.10 to 2.20 g/cm³; an intra-particulate porosity of 0 to 0.05; acircularity of 0.75 to 1.00; and an unmolten ratio of 0.00 to 0.25; andwherein the synthetic amorphous silica powder satisfies one or both ofthe conditions that it has a carbon concentration less than 2 ppm andthat it has a chlorine concentration less than 2 ppm.
 11. The method forproducing a synthetic amorphous silica powder according to claim 10,wherein, when the granulating step is a step for: hydrolyzing silicontetrachloride to produce a siliceous gel; drying the siliceous gel toturn it into a dry powder; pulverizing particles of the dry powder; andthen classifying the pulverizedly obtained particles, to obtain a silicapowder having an average particle diameter D₅₀ of 10 to 3,000 μm, theobtained synthetic amorphous silica powder has a carbon concentrationless than 2 ppm.
 12. The method for producing a synthetic amorphoussilica powder according to claim 10, wherein, when the granulating stepis a step for: hydrolyzing an organic silicon compound to produce asiliceous gel; drying the siliceous gel to turn it into a dry powder;pulverizing particles of the dry powder; and then classifying thepulverizedly obtained particles, to obtain a silica powder having anaverage particle diameter D₅₀ of 10 to 3,000 μm, the obtained syntheticamorphous silica powder has a chlorine concentration less than 2 ppm.13. The method for producing a synthetic amorphous silica powderaccording to claim 10, wherein, when the granulating step is a step for:using a fumed silica to produce a siliceous gel; drying the siliceousgel to turn it into a dry powder; pulverizing particles of the drypowder; and then classifying the pulverizedly obtained particles, toobtain a silica powder having an average particle diameter D₅₀ of 10 to3,000 μm, the obtained synthetic amorphous silica powder has a carbonconcentration less than 2 ppm and a chlorine concentration less than 2ppm.